1. Field of the Invention
The present invention relates to airframes having a combined rocket engine and ramjet or scramjet.
2. Description of the Prior Art
Ramjets provide fuel efficient propulsion of airframes at high Mach numbers. A ramjet operates under locally subsonic combustion conditions while a scramjet or supersonic combustion ramjet, operates under locally supersonic combustion conditions. To date, ramjets are in use and scramjets are currently under design but not yet field deployed. The current state of the art of scramjet technology involves ground testing in high speed wind tunnels and certain flight articles have been flown on forebodies of missiles and research airplanes. Scramjets were proposed on the X-30 high speed propulsion system but this program was cancelled before the technology was taken to fruition.
A need exists for an inexpensive high speed cruise type airframe having a fixed geometry which does not eject objects, such as staging that could injure civilians in the flight path, having a long operation range such as between 400-700 nautical miles. In order to successfully operate a ramjet engine in this environment, it is necessary to boost its speed to takeover velocities, about Mach 2 or greater, where the liquid fueled ramjet engine can operate efficiently and propel the missile for the remainder of the flight. A scramjet, or supersonic combustion ramjet, typically operates at speeds above Mach 6. Like a ramjet it requires another means to accelerate itself to operational speeds.
Chemical rocket engines provide the simplest mechanism for accelerating a platform up to takeover velocity but suffer from the deficiency of being space inefficient as a consequence of the necessity to carry oxidizer. The oxidizer is the majority of the propellant mass in a rocket-based missile which can reduce its effective range by factors between three to six times compared to ramjet engines which burn atmospheric air.
Plug nozzles were developed in the 1970's. A plug nozzle has a plume which is shaped by the external atmospheric air.
Two missile systems have been developed which utilized a solid rocket engine mounted within the combustion chamber of a ramjet. The first missile system is the French ASMP (AirSol-Moyenne-Portee') medium range air to ground missile having a range of 250 km. and a maximum speed of about Mach 3.0. The second missile is the Russian SA-6 Gainful ground to air missile which has a range of 60 km., at an altitude of 18 km. and a maximum speed of about Mach 2.8.
The U.S. Navy's Talos surface to air system, which is no longer in service, utilizes a separate rocket booster motor in conjunction with a ramjet.